The invention relates to a method and a device for correcting errors in a coordinate measuring machine having a movable measuring head and elements for guiding the measuring head in at least two different spatial directions.
According to DE 33 34 460 A1, a position of a measuring head of a coordinate measuring machine is determined interferometrically. The interferometrical value is compared by means of an evaluation device with values are supplied by a scaling, such as an optically and/or electrically scannable sequence of mechanical marks. Deviations between values supplied from the scales and the values provided by the interferometric acquisition are stored and are used in the operation of the coordinate measuring machine to correct the values supplied from the scales.
So-called guiding errors can be corrected in this way during operation of the coordinate measuring machine. Such guiding errors can be determined not only interferometrically, but also using other measuring means, such as mechanical calibrating apparatuses. A guiding error is understood as a deviation of the true coordinates of the measuring head from the coordinates which are supplied by the scales in this position. The true coordinates are dependant on the position of the guides in this case.
In the case of ideally stiff guides, such deviations between desired and actual positions can be caused by geometrical errors of the guides and/or of the scales. For example, a guide which guides a translational movement of the measuring head can have a manufacturing-induced corrugation that leads to transverse deviations of the measuring head position relative to the direction of the translation.
As an example, in order to correct for these errors, there might be defined a single measuring line (standard line) along each axis of the coordinate measuring machine, with interferometric correction values, for example, being determined in relation to this measuring line (standard method). Correction values at points which are situated in measuring volumes remote from the standard measuring lines are produced in the case of stiff guides by computational interpolation based on measured correction values of the individual standard measuring lines.
However, real coordinate measuring machines have guides which are not ideally stiff. These guides have an elasticity which is dependent on the materials and structures used. This elasticity leads to deformations of the coordinate measuring machine which cause deviations between the actual coordinates and the coordinates supplied by the elastically deformed scales, and which are therefore noticed as errors. In the case of a measuring head which is fitted at the end of a measuring arm which can be extended transverse to the direction of gravity, a flexure of the measuring arm which grows with increasing extension length will occur, for example. Such a flexure causes an elastically caused positional error of the measuring head in the direction of the flexure.
Such elastically caused errors overlap with the geometrically caused guiding errors. It is problematic here that the elastically caused errors and the geometrically caused errors can generally be a function of various influences and axes or spatial directions. The above-described standard method is directed toward the correction of geometrically caused errors, and it is therefore generally not optimal for correcting elastically caused errors. However, raising the accuracy of coordinate measuring machines also requires a correction of elastically caused errors.
DE 195 18 268 A1 discloses a method for measuring coordinates at workpieces, in the case of which the elastic bending behaviour of coordinate measuring machines is simulated by a deformation matrix. This known method delivers a good correction quality, but requires a high outlay on measuring and computing in order to determine the coefficients of the deformation matrix.